The present invention relates generally to an analysis system for use with an electric utility power system, and more particularly to an energy analysis fault detection system for detecting high impedance, low current arcing faults on the power system. Arcing faults may be caused by, for example, downed, broken, tangled or dangling power lines, trees contacting the power lines, and various overcurrent fault situations. Typically, such arcing faults are rich in non-fundamental frequency components.
Arcing faults are more difficult to detect than permanent overcurrent faults, which for instance, occur when a transformer fails. Most conventional overcurrent protection devices, such as fuses, recloser, relays and the like, have time delays which prevent a temporary fault from de-energizing the power line. Only if the overcurrent fault persists does such a protection device de-energize the power line. Some arcing faults may initialize the timing circuits of the overcurrent protection devices but, by the end of the time delay, the high impedance nature of the fault limits the fault current to a low value. Such overcurrent protection devices cannot distinguish this low fault current from the levels of current ordinarily drawn by customers; hence, the line may remain energized even though a dangerous arcing fault exists on the power line.
Other methods of detecting arcing faults have focused on the harmonic frequency content of the line current. These earlier methods compared the magnitude of line current harmonics with a predetermined reference magnitude. For instance, U.S. Pat. No. 3,308,345 to Warrington detects arcing faults by first filtering out the fundamental frequency (e.g. 60 Hertz in the United States and 50 Hertz in Europe) and its second and third harmonics. The magnitude of the remaining high harmonic frequencies, i.e., the fourth, fifth . . . harmonic frequencies, are then compared to a predetermined threshold magnitude value. Warrington measures the signals over a predetermined length of time in one frequency range. If the magnitude value of the high harmonic frequency components exceeds the reference magnitude value, the Warrington device produces a warning signal.
However, arcing often exhibits highly variable instantaneous magnitudes in many frequency ranges over time, particularly at nonharmonic frequencies near the fundamental frequency. The earlier methods concentrated only on selected high harmonic frequencies, and ignored the remainder of the frequency spectrum. These earlier methods simply failed to analyze a wide range of frequencies. Also, if the reference magnitude values were set too low, the earlier detection systems would often be too sensitive. As a result, a power line would be de-energized even though no hazardous fault existed on the line. Similarly, if the reference magnitude values were set too high or the arcing did not last long enough, the line would remain energized even though a dangerous fault existed on the power line.
Thus, a need exists for an improved high impedance energy fault detection system for electrical power transmission and distribution systems which is directed toward overcoming, and not susceptible to, these limitations and disadvantages.